It has been proposed in the past to provide a fuel injection system for a mixture-compressing spark-ignition internal combustion engine with continuous fuel injection into the engine intake manifold and an adjustable throttle valve in the manifold. A flow sensing element in the intake manifold is arranged to move in accordance with the quantity of air flowing through the manifold, and to operate a fuel-metering valve for dispensing a quantity of fuel substantially proportional or related to the quantity of air. A constant pressure valve ensures a substantially constant pressure differential across the metering valve, and as two chambers separated from each other by a diaphragm or other movable partition element. A differential pressure regulating valve also has two chambers separated by a diaphragm or other movable partition element. The first chamber of the constant pressure valve is subject to the pressure downstream of the metering valve and communicates with a fuel injection nozzle of the engine through a valve aperture controlled by the associated diaphragm. The second chamber of the constant pressure valve communicates with the first chamber of the differential pressure regulating valve, which in turn communicates with a return line via a spring loaded valve which is controlled by the diaphragm of the differential pressure regulating valve. The second chamber of the differential pressure regulating valve is subject to the pressure upstream of the metering valve.
It will be appreciated by those skilled in the art that the differential pressure at the metering valve should be kept as low as possible, since the quantity of fuel dispensed to the fuel injection jet is the product of the differential pressure and the cross-section of the dispensing orifice and, for reasons of production technology, this cross-section must not fall below a specific size. However, a low differential pressure has the disadvantage that the quantity of fuel dispensed is influenced by the internal stresses of the diaphragm of the constant pressure valve, and in such case the resultant dispensing error makes it practically impossible to dispense a quantity of fuel proportional to the quantity of induced air.